1. Download your chosen addon.
2. Extract the files to a folder on your desktop.
3. Run Rift.
4. Click on the 'addons' button on the character select screen.
5. Click on the 'open addon directory' button on the addons list screen. This will open a Windows folder window.
The path should be:
< game directory>\interface\addons
6. Copy the addon folder from your desktop into the newly opened folder window.
7. Click 'refresh' once the new addon is in the addon folder.
8. Make sure the addon is ticked and you should be all set.